1. Field of the Invention
The present invention relates to a magnetoresistive element and magnetic memory, e.g., a magnetoresistive element capable of recording information by supplying a current in two directions, and a magnetic memory using the same.
2. Description of the Related Art
The magnetoresistive effect is applied to a hard disk drive (HDD) as a magnetic memory device and presently put into practical use. The giant magnetoresistive (GMR) effect or tunneling magnetoresistive (TMR) effect is applied to a magnetic head of the HDD. Both the GMR effect and TMR effect detect a magnetic field from a magnetic medium by using a resistance change caused by an angle the magnetization directions in two magnetic layers make with each other.
Recently, various techniques have been proposed to implement a magnetic random access memory (MRAM) by using a GMR or TMR element. An example is a method of recording binary 1 or 0 in accordance with the magnetization configuration of a magnetic tunnel junction (MTJ) element, and reading the information by using the resistance change caused by the TMR effect. Various techniques have been proposed to put an MRAM using this method into practical use as well. Additionally, magnetization reversal caused by a spin-polarized current has been theoretically predicted and experimentally confirmed, and an MRAM using a spin-polarized current has been proposed. This method can switch magnetization in a magnetic layer by only supplying a spin-polarized current to the magnetic layer. If the volume of the magnetic layer is small, only smaller spin-polarized electrons need be injected. Therefore, the method is expected to advance micropatterning and reduce a current at the same time. However, the problem of thermal disturbance arises as micropatterning advances.
The magnetic anisotropic energy density must be increased in order to secure a high thermal disturbance resistance. An in-plane magnetization arrangement that has been principally studied so far generally uses the magnetic shape anisotropy. In this case, the magnetic anisotropy is secured by using the shape. This makes a switching current sensitive to the shape, and increases the variation in switching current as micropatterning advances, thus posing a problem. To increase the magnetic anisotropic energy density by using the magnetic shape anisotropy, it is possible to increase the aspect ratio of an MTJ element, increase the film thickness of a magnetic layer, or increase the saturation magnetization in the magnetic layer.
Increasing the aspect ratio of an MTJ element is unsuitable to increase the capacity because the cell area increases. Increasing the film thickness or saturation magnetization of a magnetic material is unpreferable because the value of a spin-polarized current required for magnetization reversal increases. When using not the magnetic shape anisotropy but the magnetocrystalline anisotropy in the in-plane magnetization arrangement, if a material (e.g., a Co—Cr alloy material used in a hard disk medium) having a high magnetocrystalline anisotropic energy density is used, the crystallographic axis largely disperse in the plane. This decreases the magnetoresistance (MR), or induces an incoherent precession. As a consequence, the switching current increases.
By contrast, when using the magnetocrystalline anisotropy in a perpendicular magnetization arrangement, it is possible to suppress the dispersion of the crystallographic axis that is the problem in the in-plane magnetization arrangement. For example, the crystal structure of the Co—Cr alloy material described above is the hexagonal structure, and has uniaxial magnetocrystalline anisotropy whose axis of easy magnetization is the c-axis. Therefore, the crystal orientation need only be controlled such that the c-axis is parallel to a direction perpendicular to the film surface. In the in-plane magnetization arrangement, the c-axis must be uniaxially arranged in the film surface, and the rotation of each crystal grain in the film surface rotates the crystallographic axis and disperses the uniaxial direction. In the perpendicular magnetization arrangement, the c-axis is perpendicular to the film surface. Accordingly, even when each crystal grain rotates in the film surface, the c-axis keeps being perpendicular and does not disperse.
Similarly, a perpendicular magnetization MTJ arrangement can be implemented by controlling the c-axis in the perpendicular direction in the tetragonal structure as well. Examples of a magnetic material having the tetragonal structure are an Fe—Pt ordered alloy, Fe—Pd ordered alloy, Co—Pt ordered alloy, Fe—Co—Pt ordered alloy, Fe—Ni—Pt ordered alloy, and Fe—Ni—Pd ordered alloy each having the L10 crystal structure. To use the L10 structure as a perpendicular magnetization film, however, the crystal orientation must be controlled such that the (001) plane orients. To do this, it is necessary to develop an underlying layer for controlling the crystal orientation and an annealing process for ordering in accordance with the magnetization switching method using a spin-polarized current.
It is also possible to implement the perpendicular magnetic anisotropy using the magnetic anisotropy of an interface. An example of a perpendicular magnetic film using the magnetic anisotropy of an interface is a so-called multilayer formed by repeatedly stacking magnetic layers and nonmagnetic layers. This can also suppress the dispersion of the crystallographic axis that is the problem in the in-plane magnetization arrangement. In a magnetic material having a multilayer, the perpendicular magnetic anisotropy is not ensured mainly by the magnetocrystalline anisotropy, unlike an Fe—Pt ordered alloy, and is therefore relatively hardly restricted by the crystal orientation. As a multilayer material having the perpendicular magnetic anisotropy, a system formed by alternately stacking a Co magnetic layer and a Pt nonmagnetic layer is well known.
When the magnetization reversing method using a spin-polarized current is taken into consideration, a material having a small damping constant is preferably used for a recording layer. However, if Pt of the nonmagnetic layer exists on the interface of the magnetic layer, the spin pumping effect makes the damping constant large. The magnetic layer is preferably thinned to about 0.3 to 1.0 nm from the viewpoint of the magnetic anisotropic energy density of the multilayer. However, since the thin magnetic layer enhances the spin pumping effect, the damping constant becomes large.
A high magnetoresistive ratio is necessary to increase the capacity of an MRAM from the viewpoint of read. Recently, many MTJ elements using MgO as a barrier material having a high magnetoresistive ratio have been reported, and it is regarded as important that the (100) plane of MgO orients in order to achieve a high magnetoresistive ratio. When CoFeB magnetic layers having a microcrystalline or amorphous structure are formed on both interface sides of MgO, the (100) plane orients, as is known. No multilayer using CoFeB as a magnetic layer has been reported. CoFeB having no clear crystal structure is expected to considerably decrease the perpendicular magnetic anisotropy as compared to Co having a crystal structure.
When the magnetization of the recording layer having the perpendicular magnetic anisotropy is switched using the spin transfer torque writing method, the aspect ratio of the spin transfer torque writing element can be 1. Hence, this element is also suitable for micropatterning. If magnetization reversal by a spin-polarized current is achieved in a perpendicular magnetization spin transfer torque writing element, it is possible to reduce a write current, secure a high thermal disturbance resistance of bit information, and reduce the cell area at the same time. However, the high TMR and the increase in the damping constant caused by the spin pumping effect pose problems, as described above, in forming a spin transfer torque writing element using a multilayer for a recording layer. Neither a report nor a practical method of a spin transfer torque writing element which achieves a low damping constant and a high MR ratio using a multilayer as a recording layer material has been proposed.